1. Field of the Invention
This invention relates to a method of producing crankshafts, and, in particular, to a method for producing crankshafts by which the incidence of crankshafts with defective dynamic balance is reduced so that it is as low as possible.
2. Description of Related Art
A process is known by which crankshafts, which have been profiled in a specific shape by, for instance, forging, are processed or machined to grind their counterweights to specific outer diameters and to finish their associated elements. During final processing, a measurement is taken of dynamic balance of each crankshaft in order to judge the need to make corrective balancing of the crankshaft. In this instance, a dynamic imbalance of a crankshaft is represented by a positional deviation of the center of gravity of the crankshaft from the intended center of gravity. Based on the resultant imbalance or deviation of the center of gravity, dynamic balancing is accomplished by drilling, with a drilling machine, a radial balancing bore or bores in counterweights, generally in counterweights located at the ends of the crankshaft. This processing, including the final balancing, is automated in a production-line system. Such a crankshaft producing method is known from, for instance, Japanese Unexamined Patent Publication No. 2-41730.
The apparatus described in the publication mentioned above accomplishes balancing of a crankshaft by drilling radial balancing bores to specific depths according to existing imbalance, namely a positional deviation of the center of gravity from the intended center of gravity of the crankshaft, in a specified counterweight of the crankshaft in order to offset the center of gravity in the crankshaft as close as possible to the intended center of gravity. Consequently, there are limits of correctable imbalances. That is, if a maldistribution of the mass, attributable to which a dynamic imbalance occurs, is out of a specific area of the specified counterweight in which radial balancing bores can be successfully drilled to correct the dynamic imbalance of the crankshaft, then, the dynamic balance correction cannot be accomplished by the apparatus.
Consequently, measuring of dynamic imbalance is carried out to remove defective crankshafts with dynamic imbalances which are not corrective from the production-line system. Such a defective crankshaft is properly balanced by manual drilling of radial balancing bores in a counterweight on the side where the center of gravity is deviated in the crankshaft.
For the purpose of providing an understanding of the correctable area of a counterweight for imbalance correction, reference is made to FIG. 6, which shows polar coordinates defining an imbalance correctable area of a counterweight of, for example, a crankshaft of a V-type six cylinder engine. For dynamic balancing of this crankshaft, either one of the counterweights, located at opposite ends of the crankshaft with a difference of crank angle of 180 degrees, is formed with up to four radial balancing bores. These bores have to be formed at peripheral points of the counterweight +/-30 degrees and +/-45 degrees from the center line CL of the counterweight. The maximum imbalance, which can be corrected by means of the radial balancing bores at +/-45 degree locations is 250 g/cm, whereas the maximum imbalance, which can be corrected by means of the radial balancing bores at +/-30 degree locations is 100 g/cm. That is, when manifesting a maximum imbalance which can be corrected by the four radial balancing bores formed in one end counterweight by means of vectors .alpha., .beta., .tau. and .delta. and .alpha., .beta.', .tau.' and .delta.', imbalance correction is enabled successfully with radial balancing bores formed only within the composed regions (A) and (A') by these vectors.
Unfinished crankshaft elements, such as forged crankshafts, even for the same type of engine, may have statistic tendencies of imbalances, i.e. deviations of the center of gravity from the intended center of gravity, which are attributable to production lots in which different casting molds are used and which are prepared different days. Heretofore, various processes, including grinding counterweights to their tolerable outer diameters, are applied to crankshafts uniformly for all production lots without giving consideration to the tendencies of imbalances occurring to forged crankshafts in different production lots. Because of this, if the accuracy of unfinished crankshafts, i.e. crankshaft castings, in the same production lot is low, there may be produced a significant number of finished crankshafts with defective dynamic balances which are impossible to be corrected in the final production stage. In such a case, unanticipated manual correction requirements may disrupt seriously the production schedule.